The production of molding grade polyester resins such as polyethylene terephthalate which are self-extinguishing and have high resistance to heat is of considerable commercial importance. For instance, molded articles of polyester resins are required to be self-extinguishing for many uses and are also required to endure without deterioration the heat of the molding operation as well as the heat in the environment in which they are used.
It has been observed that certain polyesters, such as polyethylene terephthalate and polybutylene terephthalate, in addition to being combustible, melt and drip readily during combustion. While the flammability of these polymers can be substantially reduced by the incorporation therein of one or more flame retardants, they are known to pose a significant fire hazard when utilized in certain fire sensitive applications.
In addition, several disadvantages accompany the use of flame retardant additives as disclosed in the prior art. Most presently available envisioned flame retardant systems operate by the release of toxic volatile decomposition products such as halogens and phosphorus compounds. Thus, the production of smoke and toxic fumes may be a problem in environments characterized by enclosed areas, such as aircraft and home interiors.
Another disadvantage associated with the use of polyester molding compositions of the prior art which incorporate various flame retardant additives heretofore proposed is that they tend to possess properties which interfere with the appearance of molded articles formed from the same in certain end use applications. For instance, many flame retardant additives heretofore proposed tend to separate from the polymeric matrix and to exude from the same over extended periods of time. The additive may become visually apparent on the surface of the molded article as a fine powder, etc.
Other disadvantages resulting from the use of flame retardant additives in the polyester composition include discoloration of the resin when exposed to use and the increased expense attributable to the presence of large quantities of the additives.
It has also been generally observed that phenolphthalein has been utilized to prepare a wide variety of polymers including polyesters. See, for example, Morgan, P. W. "Linear Condensation of Polymers from Phenolphthalein and Related Compounds", Journal of Polymer Science, volume 2, pages 437 to 459 (1964); Vinogradova, S. V.; Salazkin, S. N.; Khoruzhendo, L. N.; Kukkov, A. A. and Korshanc, V. V. "Effect of Impurities in N,N-dimethylformamide on the Breaking of the Ester Bond", Zh. Vses. Khim. Obshchest,19(3), 357 (1974); Shitikov, V. K.; Kiseley, B. A.; Stepanova, N. V.; Trofimenko, V. V.; Korshank, V. V.; and Sergeev, V. A. "Copolymers Containing Phenolphthalein", Plaste Kautsch, 21 (10), 734-6 (1974); Japanese Pat. No. 74/88928, Aug. 26, 1974, and Wagner U.S. Pat. No. 2,035,578.
Polyesters prepared from phenolphthalein and isophthalic or terephthalic acid or mixtures thereof possess fire retardant properties but have such high softening temperatures that they cannot be fabricated by conventional melt processing techniques and are not suitable molding compounds.
It is therefore an object of the present invention to provide polyesters having a melt viscosity suitable for molding applications.
It is a further object of the present invention to provide polyesters which are non-dripping when subjected to a flame even in the absence of other additive substances.
It is a further object of the present invention to provide inherently flame retardant polyesters useful in molding applications which decompose when subjected to a flame to yield relatively innocuous gaseous products.
It is a further object of the present invention to provide polyesters which are self-extinguishing when subjected to a flame even in the absence of conventional flame retardant additives.
These and other objects as well as the scope, nature, and utilization of the claimed invention will be apparent from the following detailed description and appended claims.